Quick answer
Це́рковь is feminine and irregular: в це́ркви. A cathedral is собо́р, a monastery монасты́рь.
Places
Inside
Faith and holidays
Visiting as a guest
Three rules cover almost everything. Women cover their hair with a scarf and men take their hats off. Nobody sits — there are almost no pews, and people stand throughout. And photography is usually forbidden inside: ask with Мо́жно фотографи́ровать? or look for the crossed-out camera.
Dates that differ
The Russian Orthodox Church uses the old calendar, so Christmas falls on 7 January and Easter rarely coincides with the Western date. Ста́рый Но́вый год — “Old New Year”, 14 January — is still quietly celebrated.
Typical mistakes
в це́ркве
в це́ркви
це́рковь declines irregularly.
Sitting down during a service
Stand, or step outside
Orthodox services are attended standing.
Check yourself
What do people do during an Orthodox service?